Have you checked the bios on the Msi K8n Neo2 (nforce3) for "NV/ATI Speed Up"? If this option doesn't show on your bios then it may be hidden so you will have to press "SHIFT+F2 then ALT+F3" which unlocks hidden options in Cell menu and in advanced chipset options. I reckon this could be the culprit that is overclocking your 6800gt more than you'd like.
For more info on tinkering with the bios on the K8N Neo2 check out these links:
Good Luck and let us know if this solves your issues!